The most likely problem would be that the shutoff valve at the water heater is broken and cannot be opened.

: Hi-: I just installed a Moen shower/tub mixing valve and after following all directions it's not working. The strangest thing is that I no longer have hot water at the other two faucets in the house. I think it may be trapped air somewhere so I tried shutting off the main, opened all faucets and flushed the toilet turned the main on then shut off all the faucets. I had hot water for a moment but no real pressure then it went away all together. I have water sometimes at the mixing valve but it will fade out on me. Called Moen but no real help(not plumbers I'm guessing). Please help and thanks in advance.: Take care, Bill-